Lens Module Comprising at Least One Exchangeable Optical Element

ABSTRACT

Disclosed is a lens module, especially a projection lens for semiconductor lithography, comprising at least one replaceable optical element that is disposed in a lens housing. At least one gas exchange device is positioned in an area of the replaceable optical element in such a way that a receiving zone for the replaceable optical element can be flushed when the optical element is replaced.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. National Phase Entry Under 35 U.S.C. §371 of,and claims priority under 35 U.S.C. §§ 119 and 363 to copendingPCT/EP2005/013990, filed Dec. 23, 2005 which designated the U.S. andwhich claims priority to U.S. Provisional Application No. 60/639,684,filed Dec. 23, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a lens module comprising at least oneexchangeable optical element. In this case, a lens module is understoodas meaning part of a projection lens or an illuminating lens forsemiconductor lithography or else the entire lens. The invention alsorelates to a method for flushing a receiving region of an opticalelement.

2. Description of the Related Art

In the case of high-performance lenses, as is the case for example withprojection lenses in semiconductor lithography, imaging errors after anadjustment of the lens and during use of the lens must be minimized. Tocorrect the imaging errors, an optical element that can be exchangedunder operating conditions is therefore provided.

In addition, it is advantageous to provide exchangeable optical elementsfor using the same projection exposure machine to produce differenttypes of semiconductor devices. On account of different structures (forexample perpendicular and vertical lines, via holes, honeycombstructures), different semiconductor devices lead to differentrequirements for the type of exposure in the projection exposuremachine, for example with regard to the numerical aperture or the typeof illuminating field (for example annular, dipole, quadrupole).Possible means of realizing the desired properties are, for example,filters in or near the pupil plane, which screen spatial regions of thepupil, neutral filters, which have a constant transmission or atransmission that is variable over their surface, or polarizingelements.

In this case, the exchangeable optical elements referred to may be usedboth in the projection lens and in the illuminating system of aprojection exposure machine.

However, there is the risk that freedom from contamination cannot beensured in the lens module as a result of the fitting and removal of theexchangeable optical element, entailing the introduction of gas from theoutside. This introduced gas may have the consequence that the opticalelements in the lens module of the projection exposure machine arecontaminated by (photo)chemical reaction. Furthermore, the introducedgas may impair the optical properties if it has a refractive index thatis different from that of the flushing gas of the lens module.

The present invention is therefore based on the object of providing alens module, in particular a projection lens for semiconductorlithography, with which no contamination is introduced after an opticalelement is exchanged and re-fitted, and rapid reuse is made possible.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by at least one gasexchange device being arranged in a region of the exchangeable opticalelement in such a way that a receiving region for the exchangeableoptical element can be flushed during the exchange of the opticalelement.

This means that, after the fitting of the new exchangeable opticalelement, the volume of gas in the lens module can be quickly exchangedand, as a result, further use of the lens module is possible without anyproblem after a short time.

The at least one gas exchange device according to the invention, bywhich the receiving region of the optical element is flushed during theexchange, i.e. in the opened state of the lens module, allowscontamination in the interior of the lens module during the exchange ofthe optical element to be avoided, so that much less flushing time isnecessary after the closing of the lens module. The consequently reduceddowntime of the machine has the effect that an exchange of the opticalelement is more acceptable. The risk of internal contaminations duringthe exchange of the optical element on account of reduced effectivenessof the exchangeable optical element can be significantly reduced by theflushing according to the invention of the receiving region for theoptical element.

An advantage of this solution is that the projection exposure machinecan be optimized in a very short time for this respectively chosenapplication, so that the machine can also be operated with a highthroughput in the case of small batches for a specific type of device.

It is particularly favorable if the aforementioned optimization isperformed so quickly that an optimization can be carried out after eachwafer. This enables the user to expose the same wafer twice directly insuccession, for example with vertical and horizontal structures, withouthaving to remove it from the wafer stage, which would significantlyimprove the accuracy of the spatial positioning of the two structures inrelation to each other.

This procedure, known as double exposure, has previously only been usedin conjunction with intermediate storage outside the wafer stage of thewafer exposed with one structure.

The proposed solution effectively reduces the risk of contamination bythe two mechanisms described below: during the exchange of the opticalelements, contaminations can diffuse into the lens module as a result ofit being opened for a short time. This diffusion even takes place whenthe lens module is under increased pressure in comparison with ambientpressure. Moreover, the movement of the exchangeable optical element cancause gas to be introduced from the ambience by the effects of suctionand turbulence.

Another way of introducing contamination is that of contaminationadsorbed on the exchangeable element, in particular hydrocarbons, whichmay be deposited on it during storage.

In an advantageous way, it may be provided that the at least one gasexchange device is formed as a gas inlet device in such a way that alaminar gas stream is obtained, completely or virtually preventinginward diffusion of contaminations from the ambient air during theexchange of the optical element.

In an advantageous refinement, the at least one gas inlet device has agrating device with at least one grating, with preference two or moregratings, to produce the laminar gas stream.

A laminar gas stream has the effect of avoiding turbulences, andconsequently effectively removing or keeping away from the interior ofthe lens module contaminations which could diffuse into the lens moduleduring the exchange of the optical element.

In claim 15, a further solution of the aforementioned object isspecified, a solution in which the optical element to be exchanged iskept in a gas lock; claim 22 relates to a solution in which thereceiving region of the optical element is shielded from adjacent spacesby a gas seal.

In claim 24, a method according to the invention for flushing areceiving region of an optical element is specified.

Advantageous refinements and developments of the invention are providedby the remaining subclaims. Exemplary embodiments of the invention areexplained in more detail below on the basis of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a basic representation of a projection exposure machine forsemiconductor lithography, which can be used for exposing structures onwafers coated with photosensitive materials;

FIG. 2 shows a perspective basic diagram of a mount of a lens housing inwhich an exchangeable optical element is arranged;

FIG. 3 shows a perspective representation of a gas inlet deviceaccording to the invention;

FIG. 4 shows a perspective representation of an alternative gas inletdevice according to the invention;

FIG. 5 shows a basic representation of a lens module with a number ofexchangeable optical elements for a further projection exposure machine;

FIG. 6 shows a basic representation of a lens module with a gas outletdevice in a region of the lens housing that lies opposite a push-inopening for the optical element;

FIG. 7 shows a basic representation of a lens module with a further gasinlet device on the side of the housing mount opposite the push-inopening;

FIG. 8 shows a basic representation of a lens module with a further gasinlet device on the side of the push-in opening in the housing mount;

FIG. 9 shows a basic representation of a lens module with a gas lock forkeeping the optical element;

FIG. 10 shows a basic representation of a lens module with a gas inletdevice with a valve which can be activated by means of a control device;

FIG. 11 shows a basic representation of a lens module, the inner spaceof the lens housing being designed in such a way that contamination fromthe receiving region into adjacent spaces in the lens housing is largelysuppressed;

FIG. 12 shows an alternative arrangement of the element to be exchangedin the lens module.

DETAILED DESCRIPTION

In FIG. 1, a projection exposure machine 1 for semiconductor lithographyis represented. This serves for exposing structures on a substrate whichis coated with photosensitive materials, generally consistspredominantly of silicon and is referred to as a wafer 2, for theproduction of semiconductor devices, such as for example computer chips.

The projection exposure machine 1 in this case substantially comprisesan illuminating device 3, a device 4 for receiving and exactlypositioning a mask provided with a grating-like structure, known as areticle 5, by which the later structures on the wafer 2 are determined,a device 6 for securing, advancing and exactly positioning the wafer 2and a projection lens 7.

The basic functional principle provides in this case that the structuresintroduced into the reticle 5 are exposed on the wafer 2, in particularwith a reduction in the size of the structures to one third or less ofthe original size. The requirements to be imposed on the projectionexposure machine 1, in particular on the projection lens 7, with regardto the resolution in this case lie in the range of just a fewnanometers.

Once exposure of the wafer 2 has been performed, said wafer is advanced,so that a multiplicity of individual fields, each with the structuredetermined by the reticle 5, are exposed on the same wafer 2. When theentire area of the wafer 2 has been exposed, said wafer is removed fromthe projection exposure machine 1 and subjected to a plurality ofchemical treatment steps, generally removing material by etching. Ifappropriate, a number of these exposure and treatment steps are passedthrough one after the other, until a multiplicity of computer chips arecreated on the wafer 2.

The illuminating device 3 provides a projection beam 8, for examplelight or similar electromagnetic radiation, required for the imaging ofthe reticle 5 on the wafer 2. A laser or the like may be used as thesource of this radiation. The radiation is supplied to the illuminatingdevice 3 by means of optical elements, so that the projection beam 8 hasthe desired properties with regard to diameter, polarization and thelike when it impinges on the reticle 5. An image of the reticle 5 isproduced by means of the projection beam 8 and is transmitted by theprojection lens 7 in an appropriately reduced size onto the wafer 2, asalready explained above. The projection lens 7 in this case comprises amultiplicity of individual refractive and/or diffractive elements, suchas for example lenses, mirrors, prisms, end plates or the like.

Furthermore, in the projection lens 7, at least one optical element 9which is formed as an exchangeable optical element is mounted in a mount10. The optical element 9, here a lens, is arranged in the projectionlens 7 in a pupil plane. The mount 10 with the optical element 9 is inturn mounted in a housing mount 11, which is part of a lens housing 12.In this case, the lens housing is part of the lens module or forms thelens module. The housing mount 11 in this case forms part of the outercircumference of the lens housing 12. Further such exchangeable opticalelements may likewise be provided in the projection lens 7, the opticalelement 9 in the pupil plane being assumed hereafter.

In FIG. 2, the exchangeable optical element 9 is perspectivelyrepresented. The optical element 9, mounted in the projection lens 7 ina receiving region 10′ provided for it, is connected to the mount 10 forexample by means of small supporting feet. By pushing it into thehousing mount 11 of the lens housing 12, the mount 10 is connected tothe latter by means of three fastening elements 13. The housing mount 11of the lens housing 12 comprises a ring, in particular a steel ring, inwhich a push-in opening 14 is made for the insertion of the mount 10with the optical element 9. The push-in opening 14 is made, inparticular milled, in the housing mount 11 of the lens housing 12, inorder in this way to ensure simple exchange of the optical element 9.The fastening elements 13 may be formed as adjustable fasteningelements, for example as manipulators, in order to mount the mount 10with the optical element 9 exactly and centered in the housing mount 11of the lens housing 12.

Provided in a region opposite the push-in opening 14 for the opticalelement 9 are two gas inlet devices 15, which are respectively providedto the side of a fastening element 13. In FIG. 3, one of the two gasinlet devices 15 is represented in more detail in an enlarged form. Theregions in which a gas inlet device 15 is respectively arranged in thehousing mount 11 of the lens housing 12 are identified in FIG. 2 by an“X”. During the exchange of the mount 10 together with the opticalelement 9, the mount 10 and the optical element 9 being removed from thehousing mount 11 of the lens housing 12 by way of the push-in opening14, a gas stream is supplied by way of at least one gas inlet line 16 tothe gas inlet devices 15, which produce a laminar gas stream, a gasinlet line 16 for each gas inlet device 15 being represented in FIG. 2.The gas stream introduced into the gas inlet devices 15 from outside theprojection lens 7 is supplied to the gas inlet devices 15 by a gassupply device 23 by way of the gas inlet lines 16. The laminar gasstream represented by the arrows is conducted in the direction of thepush-in opening 14, whereby a flushing of the receiving region 10′ ofthe mount 10 is performed, and so contaminations in the interior of theprojection lens 7 are avoided during the exchange of the mount 10. Oncethe mount 10 with the optical element 9 has been secured again on thefastening elements 13, the laminar flow, which flows perpendicularly inrelation to an optical axis 24 through the receiving region 10′, andconsequently transversely through the interior of the lens housing 12,and leaves at the push-in opening 14, can be ended or switched off.

The gas inlet device 15 represented in FIG. 3 has a grating device 17 atthe inlet into the receiving region 10′. Said grating device is providedwith a number of gratings 18 one behind the other and offset in relationto one another, which are only indicated here, in order to produce alaminar gas stream. In order to produce a laminar gas stream, generallyat least three gratings 18 should be provided, as shown by the basicrepresentation in FIG. 3. However, it is self-evidently also possible toprovide only one grating 18 in the grating device 17, if at least onelargely laminar gas stream can be ensured with it.

Furthermore, FIG. 3 shows the gas inlet line 16 for introducing the gasstream, which is represented here by an arrow 19. Either high-puritynitrogen or noble gas or gas mixtures of such inert gases as nitrogen ornoble gases may be used as the gas; the same gas that is already used inthe interior of the projection lens 7 for flushing the same shouldadvantageously be used. The gas pressure of the gas stream that is usedshould be regulated in such a way that a laminar gas stream is ensured.In order to conduct the gas stream from the gas inlet line 16appropriately to the grating device 17, at least one cross-bore 22 isprovided in the housing mount 11 of the lens housing 12.

Alternatively, instead of the grating device 17 with one or moregratings 18, a bore device 20 with a multiplicity of bores 21 may beprovided in a plate at the end of the bore 22 in the housing mount 11,in front of the outlet in the receiving region 10′, FIG. 4 showing saidplurality of bores arranged in a plate 20′. Since only the gratingdevice 17 has been replaced by the bore device 20, and otherwise thesame parts as in FIG. 3 are provided, the same designations have alsobeen used. The bore device 20 may also have a number of bores 21 onebehind the other and offset in relation to one another, which arearranged in plates 20′ arranged at a distance from one another.

In FIG. 5, a projection lens 7′ with a lens housing 12′ for a furtherembodiment of the projection exposure machine 1 is represented in agreatly simplified form. The device 4 for receiving and exactlypositioning the reticle 5 and the wafer 2 are indicated by dashed lines.The projection lens 7′ has, beginning at the reticle 5 and proceeding inthe radiating direction, a refractive part 31, an exchangeabledeflecting prism 32, a catadioptric part 33 with a lens 34 and anexchangeable concave deflecting mirror 35 as well as a furtherrefractive part 36 with an exchangeable end element 37. In the region ofthe exchangeable optical elements, i.e. the deflecting prism 32, theconcave deflecting mirror 35 and the end element 37, gas inlet deviceswith gas supply devices 23 and gas inlet lines 16 are arranged in thehousing mount (not represented in any more detail), whereby it ispossible during the exchange of the optical elements 32, 35 and 37 fortheir receiving regions (not represented in detail in FIG. 5) to beflushed.

In a preferred way, the gas in the inner space of the lens housing 12 isconducted in such a way that, during the exchange, it flows away throughthe push-in opening 14, as represented in FIG. 6. In the embodimentrepresented, the exchangeable element 9 is at the center of a lenshousing 12, between at least two further optical elements 9 a and 9 b.By contrast with a solution in which a gas inlet device is located atone end of the lens housing or the lens module and a gas outlet deviceis located at the other end, with preference a gas inlet device 41 or 42is respectively provided at both ends and a gas outlet device 43 isprovided in the receiving region 10′, which accommodates the element 9to be exchanged. This described arrangement of the gas inlet and outletdevices achieves the effect that penetrating contamination iseffectively removed from the inner space of the lens housing 12.

In a further advantageous embodiment, which is represented in FIG. 7, afurther gas inlet device 44 is located in the receiving region 10′,which accommodates the element 9 to be exchanged. On the side of thelens housing 12 that is facing the push-in opening 14 there is a gasoutlet device 45. It is advantageous in particular in the case of thisembodiment that, on account of the arrangement of the gas inlet andoutlet devices, a virtually laminar flow in the direction of the push-inopening 14 can form in the interior of the lens housing 12, allowingcontaminations to be efficiently removed from the lens housing 12.

Combination of the two embodiments represented in FIGS. 6 and 7 producesthe solution described in FIG. 8. In addition to the gas inlet device44, a further gas inlet device 46 is present in the vicinity of thereceiving region 10′, on the side facing the push-in opening. In thisway, further significantly increased flushing through of the receivingregion 10′ is ensured.

A slight positive pressure of 50 to 1000 Pa in relation to the ambienceusually prevails in the inner space of the lens housing 12. If anopening is then created to exchange the exchangeable element 9, thispositive pressure breaks down virtually completely, unless the gassupply is designed for a short-term delivery of extreme amounts of gas.However, this would lead to undesired side effects, such as for examplea drop in pressure in the lines. For example, with a positive pressureof 100 Pa, 4500 l/min of flushing gas would flow through an opening of100×2 mm in a 5 mm thick housing wall.

This drop in pressure must be equalized again in as short a time aspossible after introduction of the exchangeable element 9. This can takeplace by the following advantageous possibilities:

FIG. 9: the exchangeable optical element 9 is brought into a gas lock47, in which there prevails a pressure that is greater than the ambientpressure but less than or equal to the desired positive pressure in thelens housing 12. It is introduced into the gas lock 47 through the lockopening 60 and, from there, is introduced into the lens housing 12through the push-in opening 14. Optionally, a number of exchangeableelements 9 c, 9 d, 9 e may be stored in the gas lock 47. Withpreference, the gas lock 47 is flushed with the same gas as the lenshousing 12 and has its own gas inlet device 48 and a gas outlet device49.

Another embodiment is represented in FIG. 10. It is based on the idea ofequalizing the pressure loss independently of the gas supply of the lenshousing by way of the gas inlet device 41 through an additional gasinlet device 50, which is opened for the time required to equalize thedrop in pressure by a valve 51 actuated by means of a control device 54.The control may take place in a time-controlled manner, for examplestarting after a detection of the closing of the motor-operated push-inopening 14 by means of a closing sensor 70 connected to the controldevice 54, or take place in a pressure-controlled manner by means of apressure sensor 52.

Any other gas inlet device may also serve as the gas inlet device forthe gas supply; similarly, it is conceivable to close a gas outletdevice during a changing operation.

In a particularly favorable embodiment, this additional gas stream isintroduced into the receiving region 10′and switched on at the same timeas or in temporal correlation with the motorized actuation of thepush-in opening 14 in the lens housing 12. As a result, the receivingregion 10′ is continuously flushed in the direction of the exchangeopening during the changing operation. Since the gas flow is onlyrequired for the duration of the exchange, the gas flow can be amultiple of the gas flow conventionally used for flushing the lenshousing 12. In this case, it is advantageous if 95%, with preference99%, of the positive operating pressure is achieved in the lens housingwithin 5 min, with preference within 30 s, after the change of theoptical element, in particular after the closing of the push-in opening14.

FIG. 11 shows a solution by which the inward diffusion of contaminationfrom the receiving region 10′ into adjacent gas spaces 55 a and 55 b canbe restricted. Even if a higher pressure prevails in the adjacent gasspaces 55 a, 55 b than in the receiving region 10′, contamination canpenetrate into them by diffusion.

For this diffusion, the following formula applies:

$C = {C_{0} \cdot \frac{1}{e^{\frac{u \cdot L}{D}} - 1}}$

-   -   where    -   u=flow velocity of the gas in the gap    -   L=length of the gap in the direction of flow    -   D=diffusion coefficient of the gas    -   C=here concentration in the gas space 55 a    -   C₀=here concentration in the receiving region 10′

According to the invention, the contamination of the adjacent gas spaces55 a, 55 b is restricted by the gap between the adjacent gas spaces 55a, 55 b and the receiving region 10′ being provided with a suitablegeometry. For example, a peripheral gas seal 56 a, 56 b may be realizedbetween the optical elements 9 a, 9 b and the inner wall of the lenshousing 12. In this case, a gas seal is understood as meaning an atleast partially open sealing gap in which a sealing effect is achievedwith respect to the ambience by an opposing gas stream.

In FIG. 11, the preferred direction of flow of the flushing gas isindicated by the arrows 75 a,b,c,d.

Preferred here for H₂O as the contamination and N₂ as the flushing gasis a combination of gap length and flow velocity with contaminationsuppression=concentration of a gas in the gas space 55 a/concentrationof a gas in the receiving region 10′ of >10, with preference >1000.

The embodiments referred to have been shown for an element 9 located atthe center, but can also be used if the exchangeable element 9 islocated at the end of a lens housing 12, such as for example the lastoptical element of a projection lens (cf. FIG. 12), which in the presentexample is represented as a plane-parallel plate. The region of theplane-parallel plate is in this case separated from the adjoining gasspace 58 by the gas seal 57.

The exemplary embodiments described merely represent forms ofrealization that are given by way of example. It is self-evident thatfurther variants of the invention, in particular including combinationsof the exemplary embodiments or individual features of the exemplaryembodiments, are also conceivable.

1. A lens module, in particular a projection lens for semiconductorlithography, comprising at least one optical element exchangeablyarranged in a lens housing, wherein at least one gas exchange device isarranged in a region of the exchangeable optical element in such a waythat a receiving region for the exchangeable optical element can beflushed during the exchange of the optical element.
 2. A lens module asclaimed in claim 1, wherein the at least one gas exchange device isarranged in a housing mount of the lens housing.
 3. A lens module asclaimed in claim 1 or 2, wherein the at least one gas exchange device isformed as a gas inlet device in such a way that a laminar gas stream isobtained.
 4. A lens module as claimed in claim 2, wherein the at leastone gas exchange device is arranged in a region of the housing mountthat lies opposite a push-in opening for the optical element.
 5. A lensmodule as claimed in claim 4, wherein a further gas inlet device isarranged on the side of the lens housing opposite the push-in opening.6. A lens module as claimed in claim 4 or 5, wherein a further gas inletdevice is arranged on the side of the push-in opening in the lenshousing.
 7. A lens module as claimed in claim 4 or 5, wherein at leastone cross-bore, which is connected to a gas inlet line, is provided inthe housing mount.
 8. A lens module as claimed in claim 7, wherein theat least one gas exchange device, formed as a gas inlet device, has agrating device with at least one grating, which is arranged at the endof the cross-bore, in front of the outlet in the receiving region.
 9. Alens module as claimed in claim 8, wherein the grating device has atleast two gratings, to produce a laminar gas stream.
 10. A lens moduleas claimed in claim 7, wherein the at least one gas exchange device,formed as a gas inlet device, has a bore device with a multiplicity ofbores arranged at least in one plate, the bore device being arranged atthe end of the cross-bore, in front of the outlet in the receivingregion.
 11. A lens module as claimed in claim 10, wherein the boredevice has at least two plates with bores, arranged one behind theother, to produce a laminar gas stream.
 12. A lens module as claimed inclaim 9 or 11, wherein the gratings or bores arranged one behind theother are offset in relation to one another.
 13. A lens module asclaimed in claim 1, wherein the exchangeable optical element is arrangedin a pupil plane.
 14. A lens module as claimed in claim 2, wherein atleast one gas outlet device is arranged in a region of the lens housingthat lies opposite a push-in opening for the optical element.
 15. A lensmodule, in particular a projection lens for semiconductor lithography,comprising at least one optical element exchangeably arranged in a lenshousing, wherein a gas lock for keeping the at least one optical elementis present and is in connection with the inner space of the lens housingby way of a push-in opening.
 16. A lens module as claimed in claim 15,wherein the gas lock has at least one gas inlet device.
 17. A lensmodule as claimed in claim 15, wherein the gas lock has at least one gasoutlet device.
 18. A lens module as claimed in claim 15, wherein anegative pressure with respect to the inner space of the lens housingand a positive pressure with respect to the ambience prevails in the gaslock.
 19. A lens module as claimed in claim 1, wherein at least one gasinlet device is provided with a valve which can be activated by means ofa control device.
 20. A lens module as claimed in claim 19, wherein thecontrol device is in connection with a pressure sensor for measuring thepressure in the inner space of the lens housing.
 21. A lens module asclaimed in claim 19, wherein the control device is in connection with apush-in opening.
 22. A lens module, in particular a projection lens forsemiconductor lithography, comprising at least one optical elementexchangeably arranged in a lens housing, wherein the lens housing isformed in such a way that the region of the exchangeable optical elementis in connection with the remaining inner space of the lens housing byway of at least one gas seal.
 23. A lens module as claimed in claim 22,wherein, with H₂O as the contamination and N₂ as the flushing gas, thecombination of gap length and flow velocity is chosen in such a way thata contamination suppression ratio of greater than ten, is achieved. 24.A method for flushing a receiving region of an exchangeable opticalelement arranged in a lens housing, in particular an exchangeableoptical element for a projection lens in semiconductor lithography, thereceiving region being flushed with a gas during the exchange of theoptical element.
 25. A method as claimed in claim 24, wherein thereceiving region of the optical element is flushed with a laminar gasstream.
 26. A method as claimed in claim 24 or 25, wherein the flushingof the receiving region is provided in such a way that the gas streamwhich flows in from at least one gas inlet device flows into thereceiving region from a region that lies opposite a push-in opening forthe optical element.
 27. A method as claimed in one of claims 24 or 25,wherein the gas stream runs perpendicularly in relation to an opticalaxis of the optical element.
 28. A method as claimed in claim 24,wherein the gas stream through a push-in opening, which serves for thechange of the optical element, is increased in temporal correlation withthe change of the optical element.
 29. A method as claimed in claim 28,wherein the increase in the gas stream through the push-in opening isachieved by switching on an additional gas inlet device.
 30. A method asclaimed in claim 28, wherein the increase in the gas stream through thepush-in opening is achieved by an increase of the gas introduced througha gas inlet device already being used.
 31. A method as claimed in claim28, wherein the increase in the gas stream through the push-in openingis achieved by a reduction of the gas stream through a gas outletdevice.
 32. A method as claimed in claim 28, wherein the increase in thegas stream is initiated by the drop in pressure in the inner space ofthe lens housing during the change of the optical element.
 33. A methodas claimed in claim 28, wherein the gas stream is influenced by theopening state of a push-in opening during a change of the opticalelement.
 34. A method as claimed in claim 28, wherein 95%, to 99%, ofthe positive operating pressure is achieved in the lens housing withinthirty seconds to five minutes, after the change of the optical element.35. A lens module as claimed in claim 22 wherein, with H₂O as thecontamination and N₂ as the flushing gas, the combination of gap lengthand flow velocity is chosen in such a way that a contamination ratio ofat least one thousand is achieved.